A rotary atomization coating apparatus, which is conventionally known in the art, is structured so that a rotary atomizing head, which has a bell-shaped inner peripheral surface whose diameter increases from the bottom toward the tip, is rotatably mounted to a coating apparatus main body, and a centrifugal force generated by rotation is applied to paint supplied to the bottom of the inner peripheral surface of the rotary atomizing head that is rapidly rotating, thereby atomizing the paint and releasing the atomized paint.
Such rotary atomization coating apparatus performs coating of the surface of an object to be coated, by applying an electrostatic high voltage to the rotary atomizing head to charge minute particles of the atomized paint, and spraying the charged paint particles toward the object by an electrostatic field formed between the rotary atomizing head to which the electrostatic high voltage has been applied and the grounded object.
An example of the rotary atomization coating apparatus having such a structure is a coating apparatus described in Patent Document 1.
As shown in, e.g., FIG. 9, a rotary atomizing head included in such a rotary atomization coating apparatus is structured as a rotary atomizing head 101 having an inner peripheral surface 102 formed in a bottomed bell shape, and a hub portion 104, which closes a paint reservoir chamber 102a formed at the bottom of the inner peripheral surface 102, is formed on the inner peripheral surface 102.
A through hole 103 is formed in the bottom of the paint reservoir chamber 102a, and a paint supply tube 110 is inserted in the through hole 103 so that paint is supplied from the paint supply tube 110 into the paint reservoir chamber 102a. 
A plurality of paint supply holes 104a are formed in a boundary portion of the hub portion 104 with the inner peripheral surface 102, and a paint path 102b is formed in a portion of the inner peripheral surface 102, which is located on the tip side (on the left side in FIG. 9) of the hub portion 104.
Moreover, a cleaning hole 104b is formed in a central part of the hub portion 104, and a protruding portion 104c protruding in a substantially cone shape, and a paint path 104d from the protruding portion 104c toward the paint supply holes 104a are formed on the surface of the paint reservoir chamber 102a side of the central part.
When the paint is supplied into the paint reservoir chamber 102a in the state where the rotary atomizing head 101 structured as described above is being rotated at a high speed by an air motor or the like included in the rotary atomization coating apparatus, the supplied paint strikes the protruding portion 104c, and then, flows toward the outer periphery along the paint path 104d of the hub portion 104 by a centrifugal force generated by the rotation.
In this case, the paint striking the protruding portion 104c has relatively high viscosity, and thus, does not flow through the cleaning hole 104b toward the tip, but flows toward the outer periphery along the paint path 104d of the hub portion 104.
The paint, which has flown toward the outer periphery, flows to the paint path 102b through the paint supply holes 104a. 
Moreover, a paint releasing end 102c formed at the tip of the inner peripheral surface 102 has a multiplicity of serrations, and the paint, which has flown to the flow path 102b, turns into liquid ligaments at the paint releasing end 102c, and then, is released from the tip of the inner peripheral surface 102 as the liquid ligaments. The released paint in the form of the liquid ligaments are atomized and sprayed.
In this case, the paint particles released from the paint releasing end 102c try to spread toward the outer periphery by the centrifugal force. Thus, in the rotary atomization coating apparatus, shaping air 120a is blown from a shaping cap 120, which is disposed around the rotary atomizing head 101, toward a coating direction to control the spraying direction of the paint particles so that the paint particles are sprayed along a coating pattern 130.
Moreover, the rotary atomization coating apparatus is capable of supplying a cleaning solution from the paint supply tube 110 into the paint reservoir chamber 102a to clean the paint adhering to the inner peripheral surface 102 and the like with the supplied cleaning solution.
Moreover, in recent years, it has been desired to increase the paint discharge amount from one rotary atomization coating apparatus, due to demands for improved efficiency of the coating operation, and the like. However, increasing the paint discharge amount from the rotary atomizing head 101 increases the diameter of the paint that is discharged in the form of the liquid ligaments. This makes it difficult to make minute paint particles, which may affect the coating quality.
That is, in coating lines of automotive bodies and the like, it is common that a plurality of coating robots, each holding the above coating machine, are installed along the coating line so that automotive bodies and the like, which are transferred at a predetermined speed on the coating line, are coated with the paint a plurality of times by the plurality of coating robots. In order to reduce the coating cost in such coating lines, it is effective to reduce the number of times the automotive bodies are coated to reduce the number of coating robots to be installed, and to increase the transfer speed to reduce the coating time. However, these methods to reduce the coating cost cannot be implemented without increasing the paint discharge amount from the rotary atomizing heads.
However, in a mechanism of atomizing paint by the rotary atomization coating apparatuses, as shown in FIG. 10, atomization proceeds as liquid ligaments 300 released through V grooves 102d formed at an open end (the paint releasing end) of the rotary atomizing head 101 are divided. Thus, increasing only the paint discharge amount from the rotary atomizing head 101 increases the thickness of the liquid ligaments 300. This makes it difficult to atomize the paint, thereby degrading the coating film quality.
Thus, in the case of increasing the paint discharge amount, it is necessary to increase also the rotational speed of the rotary atomizing head to increase the paint releasing rate. However, increasing the rotational speed of the rotary atomizing head causes significant disturbance in the liquid ligaments 300, thereby increasing a variation in particle size distribution of atomized coating particles. That is, the particle size distribution varies from a very minute particle region of a very small particle size to a coarse particle region of a large particle size. Thus, the coating efficiency is reduced if there is a large amount of coating particles in the very minute particle region, and the coating film quality is degraded if there is a large amount of coating particles in the coarse particle region. Moreover, increasing the rotational speed of the rotary atomizing head increases the amount of atomized coating particles that are sprayed to a region around the rotary atomizing head. Thus, the pressure of the shaping air needs to be increased, which increases the amount of coating particles bouncing from the surface of an object to be coated, further reducing the coating efficiency.
Note that, for example, in a coating machine described in Patent Document 2, an annular dam (a dam portion) is provided on the inner surface of a bell cup (a rotary atomizing head) to temporarily accumulate paint therein, and the paint that overflows from the annular dam is caused to flow to a paint releasing end as a uniform thick liquid film, so that the paint can be atomized even if the paint supply amount is large. However, in this case as well, the thickness of liquid ligaments 3 (FIG. 4) increases with an increase in the paint supply amount. Thus, the rotational speed of the rotary atomizing head needs to be increased, which causes similar problems to those described above. Thus, this coating machine provides no fundamental solution.    Patent Document 1: Japanese Examined Utility Model Application Publication No. JP-Y-H06-12836    Patent Document 2: Japanese Patent Application Publication No. JP-A-2007-7506